The generally accepted practice for sonicating articles is to immerse or partially immerse those articles in a liquid bath. A sonic horn, also immersed in the bath, generates ultrasonic sound waves through the liquid medium of the bath. The articles are cleaned by the sound waves dislodging small particles from the articles.
One problem with the generally accepted configuration is recontamination. Recontamination occurs when dislodged particles redeposit themselves on articles being cleaned in a sonic bath.
Avoidance of recontamination is particularly important when ultrasonic cleaning is used in a process for sterilization. For example, commonly assigned U.S. Pat. No. 4,424,188 issued Jan. 3, 1984, which is incorporated by reference herein, discloses the use of ultrasonic irradiation in conjunction with ultraviolet irradiation to substantially reduce the number of viable microorganisms on a paperboard laminate surface, such as is used for the aseptic packaging of foodstuffs. In the '188 patent it is further disclosed that in a sterilization sequence where ultrasonic irradiation is followed by ultraviolet irradiation, the reduction in microorganisms is significantly greater than when performed in the reverse order, that is, where ultraviolet is followed by ultrasonic irradiation. The probable explanation for the significant reduction in microorganisms when ultrasonic irradiation is followed by ultraviolet irradiation is that the ultrasonic treatment dislodges and positions the microorganisms so that they are exposed to maximum ultraviolet irradiation.
In the '188 patent, the paperboard laminate to be sonicated is either fully or partially immersed in a water bath and subjected to sonication within the bath. However, using this bath configuration, there is a tendency to redistribute microorganisms over the paperboard laminate. Even sterile paperboard laminate sonicated in a used bath will become contaminated. Thus, when using the bath configuration, there is a need for constant flushing of the bath with sterile water. In addition, immersion of the paperboard laminate in a bath results in wicking (soaking up of water by capillary action) into the edge of the board possibly leading to further contamination in addition to damaging the board.